The preparation of porous membranes from resins such as polyarylsulfones (PSU) or polyarylethersulfones (PES) for applications in ultrafiltration is based on the Loeb-Sourirajan process where a polymer solution in N-methyl-2-pyrrolidone (NMP) is cast into a flat sheet or spun into a hollow fiber geometry and coagulated in water. The current processes for preparation of ultrafiltration membranes, especially those for demanding applications such as hemodialysis, also rely on incorporation of polymeric solution modifiers such as poly(vinyl pyrrolidone) (PVP) into the casting solution. The PVP is added at loadings of ˜50% by weight of the membrane-forming polymer such as PES, but the majority of the PVP is transferred to the aqueous coagulation bath after coagulation of the PES. The PVP that is solubilized in the NMP/water coagulant stream must be separated and then discarded as a material loss. The presence of PVP also complicates separation and recycle of the NMP from the waste coagulant stream since it tends to decompose under the conditions required to distill the NMP. The PVP often remains in the surface pores of the coagulated membrane and post-treatment with aqueous baths containing sodium hypochlorite (NaOCl) is commonly required to remove unwanted excess PVP from the membrane.
The loss of PVP and added process steps significantly increase the cost and complexity of the processes used to prepare ultrafiltration membranes. Thus there is a need to identify a new material for use in ultrafiltration membranes which will form a nanoporous structure when cast from polar aprotic solvents such as NMP, which forms a self-wetting hydrophilic surface, and which does not leach substantial amounts of polymeric additives into the aqueous coagulant system.